What’s the weight of the router you’re using? I had made a post earlier about the possibility of using the Dewalt 618 router, which is slightly heavier than the 611, but 2.25hp and takes 1/2" mills. Did you need to change the z-axis springs to lift the heavier router? Are you getting any deflection on the gantry?
The post about the 618: Dewalt DW618 Router